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5 <title>How to submit an LLVM bug report</title>
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10 <div class="doc_title">
11 How to submit an LLVM bug report
14 <table border="0" width="100%">
19 <li><a href="#introduction">Introduction - Got bugs?</a></li>
20 <li><a href="#crashers">Crashing Bugs</a>
22 <li><a href="#front-end">Front-end bugs</a>
23 <li><a href="#gccas">GCCAS bugs</a>
24 <li><a href="#gccld">GCCLD bugs</a>
25 <li><a href="#passes">Bugs in LLVM passes</a>
27 <li><a href="#miscompilations">Miscompilations</a></li>
28 <li><a href="#codegen">Incorrect code generation (JIT and LLC)</a></li>
32 <div class="doc_text">
33 <p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
34 <a href="http://misha.brukman.net">Misha Brukman</a></b></p>
39 <img src="Debugging.gif" alt="Debugging" width="444" height="314">
44 <!-- *********************************************************************** -->
45 <div class="doc_section">
46 <a name="introduction">Introduction - Got bugs?</a>
48 <!-- *********************************************************************** -->
50 <div class="doc_text">
52 <p>If you're working with LLVM and run into a bug, we definitely want to know
53 about it. This document describes what you can do to increase the odds of
54 getting it fixed quickly.</p>
56 <p>Basically you have to do two things at a minimum. First, decide whether the
57 bug <a href="#crashers">crashes the compiler</a> (or an LLVM pass), or if the
58 compiler is <a href="#miscompilations">miscompiling</a> the program. Based on
59 what type of bug it is, follow the instructions in the linked section to narrow
60 down the bug so that the person who fixes it will be able to find the problem
63 <p>Once you have a reduced test-case, go to <a
64 href="http://llvm.cs.uiuc.edu/bugs/enter_bug.cgi">the LLVM Bug Tracking
65 System</a>, select the category in which the bug falls, and fill out the form
66 with the necessary details. The bug description should contain the following
70 <li>All information necessary to reproduce the problem.</li>
71 <li>The reduced test-case that triggers the bug.</li>
72 <li>The location where you obtained LLVM (if not from our CVS
76 <p>Thanks for helping us make LLVM better!</p>
80 <!-- *********************************************************************** -->
81 <div class="doc_section">
82 <a name="crashers">Crashing Bugs</a>
84 <!-- *********************************************************************** -->
86 <div class="doc_text">
88 <p>More often than not, bugs in the compiler cause it to crash - often due to an
89 assertion failure of some sort. If you are running <tt><b>opt</b></tt> or
90 <tt><b>analyze</b></tt> directly, and something crashes, jump to the section on
91 <a href="#passes">bugs in LLVM passes</a>. Otherwise, the most important
92 piece of the puzzle is to figure out if it is the GCC-based front-end that is
93 buggy or if it's one of the LLVM tools that has problems.</p>
95 <p>To figure out which program is crashing (the front-end,
96 <tt><b>gccas</b></tt>, or <tt><b>gccld</b></tt>), run the
97 <tt><b>llvm-gcc</b></tt> command line as you were when the crash occurred, but
98 add a <tt>-v</tt> option to the command line. The compiler will print out a
99 bunch of stuff, and should end with telling you that one of <tt><b>cc1</b></tt>,
100 <tt><b>gccas</b></tt>, or <tt><b>gccld</b></tt> crashed.</p>
104 <li>If <tt><b>cc1</b></tt> crashed, you found a problem with the front-end.
105 Jump ahead to the section on <a href="#front-end">front-end bugs</a>.</li>
107 <li>If <tt><b>gccas</b></tt> crashed, you found a bug in <a href="#gccas">one
108 of the passes in <tt><b>gccas</b></tt></a>.</li>
110 <li>If <tt><b>gccld</b></tt> crashed, you found a bug in <a href="#gccld">one
111 of the passes in <tt><b>gccld</b></tt></a>.</li>
113 <li>Otherwise, something really weird happened. Email the list with what you
114 have at this point.</li>
120 <!-- ======================================================================= -->
121 <div class="doc_subsection">
122 <a name="front-end">Front-end bugs</a>
125 <div class="doc_text">
127 <p>If the problem is in the front-end, you should re-run the same
128 <tt>llvm-gcc</tt> command that resulted in the crash, but add the
129 <tt>-save-temps</tt> option. The compiler will crash again, but it will leave
130 behind a <tt><i>foo</i>.i</tt> file (containing preprocessed C source code) and
131 possibly <tt><i>foo</i>.s</tt> (containing LLVM assembly code), for each
132 compiled <tt><i>foo</i>.c</tt> file. Send us the <tt><i>foo</i>.i</tt> file,
133 along with a brief description of the error it caused.</p>
137 <!-- ======================================================================= -->
138 <div class="doc_subsection">
139 <a name="gccas">GCCAS bugs</a>
142 <div class="doc_text">
144 <p>If you find that a bug crashes in the <tt><b>gccas</b></tt> stage of
145 compilation, compile your test-case to a <tt>.s</tt> file with the
146 <tt>-save-temps</tt> option to <tt><b>llvm-gcc</b></tt>. Then run:</p>
149 <b>gccas</b> -debug-pass=Arguments < /dev/null -o - > /dev/null
152 <p>... which will print a list of arguments, indicating the list of passes that
153 <tt><b>gccas</b></tt> runs. Once you have the input file and the list of
154 passes, go to the section on <a href="#passes">debugging bugs in LLVM
159 <!-- ======================================================================= -->
160 <div class="doc_subsection">
161 <a name="gccld">GCCLD bugs</a>
164 <div class="doc_text">
166 <p>If you find that a bug crashes in the <tt><b>gccld</b></tt> stage of
167 compilation, gather all of the <tt>.o</tt> bytecode files and libraries that are
168 being linked together (the "<tt><b>llvm-gcc</b> -v</tt>" output should include
169 the full list of objects linked). Then run:</p>
172 <b>llvm-as</b> < /dev/null > null.bc
173 <b>gccld</b> -debug-pass=Arguments null.bc
176 <p>... which will print a list of arguments, indicating the list of passes that
177 <tt><b>gccld</b></tt> runs. Once you have the input files and the list of
178 passes, go to the section on <a href="#passes">debugging bugs in LLVM
183 <!-- ======================================================================= -->
184 <div class="doc_subsection">
185 <a name="passes">Bugs in LLVM passes</a>
188 <div class="doc_text">
190 <p>At this point, you should have some number of LLVM assembly files or bytecode
191 files and a list of passes which crash when run on the specified input. In
192 order to reduce the list of passes (which is probably large) and the input to
193 something tractable, use the <tt><b>bugpoint</b></tt> tool as follows:</p>
196 <b>bugpoint</b> <input files> <list of passes>
199 <p><tt><b>bugpoint</b></tt> will print a bunch of output as it reduces the
200 test-case, but it should eventually print something like this:</p>
204 Emitted bytecode to 'bugpoint-reduced-simplified.bc'
206 *** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm
209 <p>Once you complete this, please send the LLVM bytecode file and the command
210 line to reproduce the problem to the llvmbugs mailing list.</p>
214 <!-- *********************************************************************** -->
215 <div class="doc_section">
216 <a name="miscompilations">Miscompilations</a>
218 <!-- *********************************************************************** -->
220 <div class="doc_text">
222 <p>A miscompilation occurs when a pass does not correctly transform a program,
223 thus producing errors that are only noticed during execution. This is different
224 from producing invalid LLVM code (i.e., code not in SSA form, using values
225 before defining them, etc.) which the verifier will check for after a pass
226 finishes its run.</p>
228 <p>To debug a miscompilation, you should choose which program you wish to run
229 the output through, e.g. C backend, the JIT, or LLC, and a selection of passes,
230 one of which may be causing the error, and run, for example:</p>
233 <b>bugpoint</b> -run-cbe [... optimization passes ...] file-to-test.bc
236 <p><tt>bugpoint</tt> will try to narrow down your list of passes to the one pass
237 that causes an error, and simplify the bytecode file as much as it can to assist
238 you. It will print a message letting you know how to reproduce the resulting
243 <!-- *********************************************************************** -->
244 <div class="doc_section">
245 <a name="codegen">Incorrect code generation</a>
247 <!-- *********************************************************************** -->
249 <div class="doc_text">
251 <p>Similarly to debugging incorrect compilation by mis-behaving passes, you can
252 debug incorrect code generation by either LLC or the JIT, using
253 <tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try
254 to narrow the code down to a function that is miscompiled by one or the other
255 method, but since for correctness, the entire program must be run,
256 <tt>bugpoint</tt> will compile the code it deems to not be affected with the C
257 Backend, and then link in the shared object it generates.</p>
259 <p>To debug the JIT:</p>
262 <b>bugpoint</b> -run-jit -output=[correct output file] [bytecodefile]
265 <p>Similarly, to debug the LLC, one would run:</p>
268 <b>bugpoint</b> -run-llc -output=[correct output file] [bytecodefile]
271 <p>At the end of a successful <tt>bugpoint</tt> run, you will be presented
272 with two bytecode files: a <em>safe</em> file which can be compiled with the C
273 backend and the <em>test</em> file which either LLC or the JIT
274 mis-codegenerates, and thus causes the error.</p>
276 <p>To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do
281 <li>Regenerate the shared object from the safe bytecode file:<br>
284 <b>llc</b> -march=c safe.bc -o safe.c<br>
285 <b>gcc</b> -shared safe.c -o safe.so
288 <li>If debugging LLC, compile test bytecode native and link with the shared object:<br>
291 <b>llc</b> test.bc -o test.s -f<br>
292 gcc test.s safe.so -o test.llc<br>
293 ./test.llc [program options]
296 <li>If debugging the JIT, load the shared object and supply the test
300 <b>lli</b> -load=safe.so test.bc [program options]
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